Wind turbine blades and thereby also the root region thereof are often made by assembling two blade halves essentially corresponding to the suction side and the pressure side, respectively, along the chord plane. However, the blades may also be moulded in their entirety by so-called hollow moulding.
The root region comprises layers of fibres forming an outer layer and an inner layer between which fastening members in the form of bushings are placed. Separately formed inserts may be placed between each pair of adjacent bushings, whereby the bushings are mutually separated by the inserts. The known inserts are made of glass fibres embedded in a suitable resin.
A potential problem in connection with wind turbine blades is load transfer from the fibre composite structure of the root region to the hub of the wind turbine. The connection and transfer of loads from the blade to the hub is inter alia provided by mounting the blade to the hub by mounting bolts into the bushings placed in the root or by means of nuts mounted onto the stud bolts mounted into the bushings. In case the number of bolts and thereby the number of bushings has to be increased to handle a given load, the remaining area of the fibre composite material between the bushings is reduced. This may result in the root connection being insufficiently supported to withstand the loads, whereby the connection between the blade root and the hub may fail since the bushings are insufficiently retained in the composite material and thus pulled out of the composite material of the root region. This is especially a problem when long and thereby heavy blades are to be used. In principle, it is also possible to increase the area of the bushings so that the bushings are located along the entire circumference of the root. However, such a solution adds weight to the root region and further increases costs. Further, it is a restraint that the bushings of course only can extend along the entire circumference of the blade.
WO 2006/070171 discloses a method of manufacturing a wind turbine shell member provided with an incorporated fastening member near the root. The document mentions that it may be advantageous for thin blade shells in using fastening members with an oval cross section and the major axis oriented parallel to the surface of the blade shell. If the oval fastening members have to be closely spaced, the fastening members may be slightly skewed but still substantially parallel to the surface of the blade shell.
DE 10 2008 021498 A1 discloses a method for manufacturing a blade connection of a rotor blade for a wind turbine system, wherein fastening members are arranged equidistantly spaced on an arc. The fastening members may have a trapezoidal or square cross-section.
EP 2 138 716 A1 discloses a blade insert connected in the lamination of a wind turbine blade. The inserts may be separated by separators made from fiberglass or foam.
WO 2010/018225 provides a method of manufacturing a wind turbine blade comprising a steel wire or steel fibre-reinforced polymer matrix. However, the document does not address the problem of how the root region is to be designed to withstand extreme loads in the connection between the blade root and the hub.